1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method positioning one of plural cartridges mounted to a developing unit at a development position and performing a printing operation in a specific color using toner in the cartridge.
2. Description of the Related Art
Heretofore, there have been widely known image forming apparatuses adapted to form an image using a plurality of developer cartridges. For instance, Japanese Unexamined Patent Publication No. 2003-215862 discloses a color image forming apparatus including a rotary developing unit having four developer cartridges radially arranged about a rotating shaft. In this apparatus, the rotating shaft is driven into rotation for selectively positioning one of the four developer cartridges at a place opposite to a latent image carrier such as a photosensitive member so as to develop a latent image on the latent image carrier. Subsequently, the developed image is transferred to an intermediate transfer medium. The developing process and the transfer process are repeated the same way as the above while switching from one developer cartridge to another, whereby toner images of plural colors are superimposed on top of each other to form a color image.
In the aforementioned color image forming apparatus, the four developer cartridges contain therein toners of individually different colors (yellow, cyan, magenta and black) so as to perform the printing operation for color image. Therefore, there occurs a problem that in a case where the above image forming apparatus is used for printing monochromatic images, the above image forming apparatus runs out of the black toner faster than an image forming apparatus dedicated to monochromatic image printing. In order to overcome such a problem, there has been proposed an apparatus wherein black developer cartridge(s) is(are) mounted to place(s) where the yellow, cyan and/or magenta developer cartridge(s) are mounted, and wherein when one of the black developer cartridges runs out of the black toner, the developer cartridge is switched to another black developer cartridge so as to permit the continuation of the monochromatic printing operation (see, for example, Japanese Unexamined Patent Publication No. 2002-351190).